1. Field of the Invention
The present invention relates to a head positioning apparatus and, more particularly, to such an apparatus for a helical-scan video tape recorder.
2. Description of the Prior Art
Helical-scan video tape recorders, or HVTR's, record tracks on a magnetic tape using two magnetic heads spaced 180.degree. apart on a rotating disc or drum. The tape is wrapped around about one-half of the periphery of the drum so that each head records or reproduces one track on the tape during about one-half of a revolution of the head. While one head is in contact with the tape, the other is in a "flyback" position. The geometry of the HVTR recording-reproducing system results in the magnetic impression of a plurality of tracks on the tape at an angle to the direction of tape travel.
Accurate reproduction of those tracks requires that each head trace the particular track being reproduced without tracing either adjacent track. In known HVTR's the heads are each mounted to the drum by an electro-mechanical transducer commonly known as a bi-morph leaf. During playback, a voltage can be applied to the bi-morph leaf to deflect it and position the head as the head mounted on the bi-morph leaf scans the tape. If the deflection of the bi-morph leaf is properly controlled, the head mounted thereon will accurately trace the track being reproduced. When the tape speed during reproduction is different from the tape speed during recording, the bi-morph leaf must be deflected a substantial amount. Under such circumstances, mechanical hysteresis can prevent the bi-morph leaf from returning during flyback to a position in which it is properly located for tracing the next track on the tape.
In the prior art a signal has been applied to the bi-morph leaf during flyback to "erase" the mechanical hysteresis. The hysteresis-erase signal usually comprises a decaying, oscillating voltage that flexes the bi-morph leaf back and forth during flyback. The bi-morph leaf is supported to return to its proper position under the influence of the hysteresis-erase signal as the end of flyback approaches so that it can be accurately deflected by a tracking correction signal during the next trace of the tape. Since the period available for erasing the hysteresis is limited, the hysteresis-erase signal has to be of short duration.
The tracking correction signal used to deflect the bi-morph leaf to position the head while it traces the tape must deflect the bi-morph leaf in one direction when playback is faster than the speed at which the tape was recorded and in the other direction when playback is slower than the speed at which the tape was recorded. It has been found that one hysteresis-erase signal of short duration that may be efective when the bi-morph leaf has been deflected in one direction while tracing the tape may not be effective to erase the mechanical hysteresis that occurs when the bi-morph leaf has been deflected in the other direction.